1. Field of the Invention
The present invention is within the field of electrographical printing devices. More specifically, the invention relates to an apertured printhead structure brought into cooperation with a particle source to modulate a stream of toner particles from the particle source through the apertured printhead structure.
2. Description of the Related Art
U.S. Pat. No. 5,036,341 granted to Larson discloses a direct electrostatic printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals. The Larson patent discloses a method in which an electrode matrix, arranged between a back electrode and a rotating particle carrier, generates a pattern of electrostatic fields which, due to control in accordance with an image information, modulate a transport of toner particles toward the back electrode. An electrostatic field on the back electrode attracts the toner particles from the surface of the particle carrier to create a particle stream toward the back electrode. The particle stream is modulated by voltage sources which apply an electric potential to selected individual control electrodes to create electrostatic fields which either permit or restrict the transport of toner particles from the particle carrier through the electrode matrix, In effect, these electrostatic fields "open" or "close" selected apertures in the electrode matrix to the passage of toner particles by influencing the attractive force from the back electrode. The modulated stream of charged toner particles allowed to pass through the opened apertures impinges upon a print-receiving medium interposed in the particle stream to provide line-by-line scan printing to form a visible image.
An electrode matrix for use in direct electrostatic printing devices may take on many designs, such as a lattice of intersecting wires arranged in rows and columns, or a screen-shaped, apertured printed circuit. Generally, the matrix is formed of a thin, flexible substrate of electrically insulating material, such as polyimide, provided with a plurality of apertures and overlaid with a printed circuit of control electrodes arranged in connection to the apertures, such that each aperture is surrounded by an individually addressable control electrode.
An essential requirement of the aforementioned method is to maintain a constant, uniform gap distance between the electrode matrix and the particle carrier. The gap distance can vary from machine to machine because it is determined by a combination of independent factors such as manufacturing variations in the size and placement of the particle carrier and the electrode matrix, as well as the thickness of the toner layer on the particle carrier.
U.S. Pat. No. 5,666,147, also granted to Larson, discloses improved means for maintaining a constant minimal gap between the electrode matrix and the particle carrier, while providing a uniform toner layer on the surface of the particle carrier. According to that patent, a spacer is mounted on the electrode matrix on the side facing the particle carrier to engage the carrier on it, and the portion of the array supporting the spacer can move slightly radially towards and away from the carrier to accommodate imperfections in the carrier surface and variations in the toner layer thickness. The gap distance is thus maintained at a constant value according to the thickness of the spacer, independent of the thickness of the particle layer.
However, even if undesired distance variations of the gap between the electrode matrix and the particle carrier are considerably reduced by spacers, there is still a need for improving the material composition and the configuration of the spacers, to obtain required properties such as, for example, a combination of high hardness, low surface roughness, low friction and chemical inertness.